Comparative Analysis of Three Casting Processes for Aluminum Alloy Shock Toweras

Choosing the Right Casting Process for Aluminum Shock Towers: A Data-Driven Comparison for Mass Production

This technical summary is based on the academic paper "Comparative Analysis of Three Casting Processes for Aluminum Alloy Shock Towers" by Zhang You-guo, Wang Xue-feng, and Huang Zhi-gang, published in Foundry (2019).

Keywords

• Primary Keyword: Aluminum Shock Tower Casting
• Secondary Keywords: High-Vacuum HPDC, Investment Casting, Sand Casting, Self-Piercing Riveting (SPR), Automotive Lightweighting, AlSi10MnMg

Executive Summary

• The Challenge: To select the optimal casting process for a lightweight, one-piece aluminum shock tower for an electric vehicle, balancing performance, cost, and manufacturability from prototype to mass production.
• The Method: A comparative analysis of three casting processes—investment casting (ZL114A), sand casting (AlSi7Mg), and high-vacuum high-pressure die casting (HPDC) (AlSi10MnMg)—was conducted throughout the component's development cycle.
• The Key Breakthrough: The material's elongation is the most critical factor for ensuring the integrity of Self-Piercing Riveting (SPR) connections; high-vacuum HPDC produced parts with the highest elongation (14.1%), resulting in defect-free SPR joints.
• The Bottom Line: For mass-produced structural aluminum components requiring high-integrity mechanical joining, high-vacuum HPDC is the superior manufacturing process due to its ability to produce parts with excellent ductility, consistency, and production efficiency.

The Challenge: Why This Research Matters for HPDC Professionals

The push for electric vehicles (EVs) has intensified the demand for automotive lightweighting to extend battery range and improve performance. Replacing traditional multi-part steel shock towers with a single-piece cast aluminum component offers significant weight savings (up to 40%, as noted in the paper). However, the journey from initial prototype to mass production presents a significant challenge. Engineers must select a manufacturing process that not only meets evolving performance requirements (strength, stiffness, crash safety) but is also scalable and cost-effective. A poor choice in the early stages can lead to costly redesigns, production delays, and, most critically, component failure in areas like mechanical joints. This research directly addresses the critical decision-making process by providing a head-to-head comparison of three common casting methods used at different stages of automotive development.

The Approach: Unpacking the Methodology

The study systematically evaluated three distinct casting processes, each corresponding to a phase in the automotive development timeline (Mule, DV, PV). All tested shock towers underwent a T7 heat treatment to optimize mechanical properties.

Method 1: Investment Casting (Lost Wax Casting)
- Material: ZL114A aluminum alloy.
- Process: A low-pressure gravity casting method. Ideal for small quantities and rapid prototyping, allowing for flexible design changes.
- Characteristics: Short development cycle but low dimensional accuracy, high wall thickness variation (0.5-1.5 mm), and slow production speed.

Method 2: Low-Pressure Sand Casting
- Material: AlSi7Mg0.3 aluminum alloy.
- Process: Molten aluminum is cast into a single-use sand mold under low pressure.
- Characteristics: Also offers a short development cycle and supports design changes. Provides better dimensional stability than investment casting but suffers from a slow production cycle.

Method 3: High-Vacuum High-Pressure Die Casting (HPDC)
- Material: AlSi10MgMn aluminum alloy.
- Process: Molten aluminum is injected into a steel mold under high pressure (2700 t) and high vacuum (<50 mbar) to minimize porosity.
- Characteristics: Designed for mass production. Offers excellent part-to-part consistency and high efficiency but has a long development lead time and is less flexible for design changes.

The Breakthrough: Key Findings & Data

The investigation revealed a clear hierarchy of performance among the three processes, with material ductility (elongation) emerging as the decisive factor for production viability.

Finding 1: HPDC Delivers Superior Microstructure and Ductility

The casting process directly influenced the final microstructure and, consequently, the mechanical properties. As shown in Figure 5, the high cooling rate of the steel die in the HPDC process resulted in the finest, most uniform grain structure. In contrast, investment casting produced the coarsest grains.

This microstructural difference had a profound impact on mechanical properties. According to Table 1, while the investment-cast ZL114A parts had the highest tensile strength (avg. 239 MPa), they exhibited the lowest elongation (avg. 6.5%). The sand-cast AlSi7Mg parts showed moderate strength and elongation (avg. 11.9%). Critically, the HPDC AlSi10MnMg parts, despite having the lowest tensile strength (avg. 202.7 MPa), delivered the highest average elongation at 14.1%, comfortably exceeding the 10% minimum requirement for the production component.

Finding 2: High Elongation is Non-Negotiable for Reliable SPR Connections

The ability to join the cast shock tower to the surrounding body structure using Self-Piercing Riveting (SPR) was a critical performance requirement. The study found that material elongation was directly correlated with SPR joint quality.

As seen in Figure 7, the low-elongation (6.5%) investment-cast parts exhibited visible cracking around the SPR joint, making them unsuitable for production. The sand-cast parts (11.9% elongation) performed better but still failed one of the four SPR connection tests (Table 5). Only the high-elongation (14.1%) HPDC parts passed all four SPR connection tests without any signs of cracking or defects, demonstrating superior joint integrity and reliability essential for vehicle safety and assembly.

Practical Implications for R&D and Operations

• For Process Engineers: This study suggests that for structural components requiring mechanical fastening like SPR, prioritizing alloys and processes that maximize elongation is critical. The data indicates that an elongation below 10% presents a significant risk of joint failure.
• For Quality Control Teams: The data in Table 1 and Figure 5 of the paper illustrates the direct link between microstructure and mechanical properties. This reinforces the importance of microstructural analysis as a quality control metric to predict final part performance, especially ductility.
• For Design Engineers: The findings indicate that the choice of joining technology (e.g., SPR vs. FDS/welding) should be considered in conjunction with the manufacturing process and material selection. For parts intended for mass production with SPR, designing for a process like high-vacuum HPDC that guarantees high elongation is a key consideration from the start.

Paper Details

Comparative Analysis of Three Casting Processes for Aluminum Alloy Shock Towers

1. Overview:

• Title: 铝合金减震塔的三种铸造工艺对比分析 (Comparative Analysis of Three Casting Processes for Aluminum Alloy Shock Towers)
• Author: 张友国 (Zhang You-guo), 王雪峰 (Wang Xue-feng), 黄智钢 (Huang Zhi-gang)
• Year of publication: 2019
• Journal/academic society of publication: 铸造 (Foundry), Vol. 68, No. 10
• Keywords: 铝合金 (aluminum alloy); 熔模铸造 (investment casting); 砂型铸造 (sand casting); 高真空高压铸造 (high vacuum assisted high pressure casting); 减震塔 (shock tower)

2. Abstract:

This paper introduces three different casting processes used in the positive development and validation of an aluminum alloy shock tower: investment casting, sand casting, and high vacuum assisted high pressure casting. It comparatively analyzes the differences in microstructure, mechanical properties, and connection performance of parts cast using these three processes and their corresponding materials. Finally, based on the application in the actual production and use process, the advantages and disadvantages of the three casting processes are summarized.

3. Introduction:

For pure electric vehicles, new components such as power batteries not only significantly increase weight but also demand higher safety performance than traditional fuel vehicles. Consumer preferences and government subsidy policies require electric vehicle manufacturers to continuously explore ways to increase cruising range. With battery energy density facing a bottleneck, using lightweight materials to improve electrical consumption economy has become the sole option for many manufacturers. Aluminum alloy, with a lower density than iron and a lower cost than carbon fiber, is widely used in the body and chassis components of electric vehicles. The application of cast aluminum alloys in mass-produced vehicle body structural parts is still relatively limited in China, with a lack of accumulated experience. This paper details the development of an integrated cast aluminum shock tower, which replaces multiple sheet metal parts, offering significant weight reduction and improvements in dimensional accuracy and performance. The development process of a certain pure electric vehicle's aluminum alloy shock tower is shown, evolving through TG0, TG1, and TG2 stages, each involving optimization of layout, safety performance, modality, and process.

4. Summary of the study:

Background of the research topic:

The development of electric vehicles necessitates lightweighting to improve range and performance. Integrated cast aluminum shock towers are a cost-effective solution for reducing weight and consolidating parts compared to traditional steel assemblies.

Status of previous research:

The application of cast aluminum structural parts is less mature in the domestic Chinese market. Foreign research is extensive, with examples like BMW using high-vacuum die-cast shock towers for a 40% weight reduction. Research by Lee et al. has studied the effects of thixoforming defects on the mechanical properties of A357 shock towers. High-vacuum die casting is widely recognized as an effective method to improve the internal porosity of cast aluminum structural parts for mass production.

Purpose of the study:

To compare three different casting processes (investment casting, sand casting, and high-vacuum HPDC) used during the phased development of an aluminum shock tower for a pure electric vehicle. The study aims to evaluate the resulting components' microstructure, mechanical properties, and connection performance to determine the optimal process for each development stage and for final mass production.

Core study:

The study analyzed shock towers produced by three methods:
1. Investment Casting: Using ZL114A alloy for early-stage prototypes.
2. Sand Casting: Using AlSi7Mg0.3 alloy for engineering validation samples.
3. High-Vacuum HPDC: Using AlSi10MgMn alloy for product validation and mass production.

All samples underwent a T7 heat treatment. The study involved tensile testing, microstructural analysis (metallography), X-ray inspection for internal defects, and connection performance tests (press-in nut and Self-Piercing Riveting).

5. Research Methodology

Research Design:

A comparative study was designed to evaluate parts produced by three distinct casting processes at different stages of an automotive component development cycle. The evaluation criteria included mechanical properties (tensile strength, yield strength, elongation), microstructure, internal quality (porosity), and joining performance (press-in nut and SPR).

Data Collection and Analysis Methods:

• Mechanical Properties: Tensile tests were conducted on samples taken from three locations (top, middle, bottom) of the shock tower, following ISO 6892-1/DIN 50125 standards.
• Microstructural Analysis: Samples from the same location on each type of part were examined using scanning electron microscopy to compare grain size and structure.
• Internal Defect Analysis: Industrial X-ray inspection was performed according to ASTM E505 standards to assess porosity levels in critical areas.
• Connection Performance: Ejection force and maximum torque tests were performed on M8 press-in nuts. Self-Piercing Riveting (SPR) tests were conducted with four different material/thickness combinations, followed by visual inspection and cross-sectional analysis (dissection) to measure key joint parameters.

Research Topics and Scope:

The research is focused on a single component: an integrated aluminum alloy shock tower for a pure electric vehicle. The scope covers the comparison of three specific casting processes and their associated alloys (ZL114A, AlSi7Mg0.3, AlSi10MnMg) as they relate to manufacturability, performance, and suitability for different development phases from prototype to mass production.

6. Key Results:

Key Results:

• Mechanical Properties (Table 1): Investment casting (ZL114A) yielded the highest strength but the lowest elongation (avg. 6.5%). Sand casting (AlSi7Mg) provided intermediate properties with an average elongation of 11.9%. High-vacuum HPDC (AlSi10MnMg) had the lowest strength but the highest elongation (avg. 14.1%).
• Microstructure (Figure 5): HPDC produced the finest and most dense grain structure due to rapid solidification. Investment casting produced the coarsest structure.
• X-Ray Inspection (Figure 6): All three processes produced parts that met the internal quality requirements (<1% porosity) in the critical 'A' zone.
• SPR Connection Performance (Table 5, Figure 7, Figure 8): The low-elongation investment cast parts showed cracking at the SPR joint. The sand cast parts failed one of the four SPR tests. The high-elongation HPDC parts passed all SPR connection tests successfully, showing no defects.
• Economic Comparison (Table 6): Investment casting has the highest piece price (€6000) and moderate tooling cost. Sand casting has a lower piece price (€4000) and the lowest tooling cost. HPDC has the lowest piece price (€200) but the highest tooling cost (€400,000), making it suitable only for high-volume production.

Figure Name List:

• 图1 传统钢减震塔和铸铝减震塔结构及重量对比
• 图2 铸铝减震塔从TG0、TG1到TG2的开发设计过程演变
• 图3 铸铝减震塔1#、2#、3#三处取样位置
• 图4 三种铸造工艺减震塔在三个不同位置的试片应力应变曲线
• 图5 三种铸造工艺减震塔的微观组织对比
• 图6 三种铸造工艺减震塔顶部区域的X光检测对比
• 图7 三种铸造工艺减震塔的SPR连接外观图
• 图8 三种铸造工艺减震塔不同SPR连接组合的剖检图

7. Conclusion:

1. A direct relationship exists between the material's grain size, elongation, and SPR connection reliability. Finer grains lead to higher elongation, which is crucial for preventing cracking during the SPR process. Materials with elongation below 10% are at high risk of SPR joint failure.
2. For new cast aluminum component development, sand casting with AlSi7Mg is recommended for small-batch trial production. If investment casting with ZL114A must be used, alternative joining methods like FDS or welding (which are less sensitive to material elongation) should be considered. For mass production, high-vacuum HPDC with AlSi10MnMg is the recommended choice to balance performance, cost, and development cycle.
3. Based on the performance of different materials and processes, the performance requirements for a mass-produced cast aluminum shock tower are defined as: Yield Strength (Rp0.2) > 120 MPa, Tensile Strength (Rm) > 180 MPa, and Elongation > 10%.

8. References:

• [1] 汪学阳,黄志垣,廖仲杰,等.高真空压铸铝合金减震塔工艺开发及应用[J]. 特种铸造及有色合金,2018,38(8):860-863.
• [2] HARTLIEB M. Aluminum alloys for structural die casting [J]. Die Casting Engineer, 2013(5): 40-43.
• [3] LEE Sang Yong. Effects of thixoforming defects on the stress-strain curves of aluminum structural parts for automobile [C] //Solid State Phenomena (1662-9779), 2008: 743.
• [4] WO L A. Vacuum die casting: Its benefits and guidelines [C] //Transactions of 16th International NADCA Congress and Exposition, Paper No.T91-071, Detroit, MI, USA, 1991.
• [5] JOHANN E. Vacuum die casting technology for automotive components [C] //Ohio: NADCA, 2006: T06-62.
• [6] 杨斌.3D打印技术在熔模精密铸造样件上的应用研究[D]. 镇江:江苏大学,2018.
• [7] 阮明,刘海峰,姚红,等. 大型薄壁铝合金减震塔砂型铸造技术研究[J]. 铸造,2017,66(4):327-331, 336.
• [8] 陈正周,宋朝辉,王菊清. 薄壁多筋铝合金腔体低压铸造工艺[J]. 铸造,2019,68(6): 613-617.
• [9] 朱必武. AlSi10MnMg薄壁铝合金件压铸流动行为及其组织力学性能[D]. 长沙:湖南大学,2013.
• [10]陈瑞,许庆彦,郭会廷,等. Al-7Si-Mg铸造铝合金拉伸过程应力-应变曲线和力学性能的模拟[J]. 铸造,2016,65(8):737-743.

Expert Q&A: Your Top Questions Answered

Q1: Why was material elongation identified as more critical than tensile strength for this specific component?

A1: While tensile strength is important for overall component durability, the study found that elongation was the limiting factor for manufacturability at the vehicle assembly stage. The shock tower required joining to the body via Self-Piercing Riveting (SPR), a cold-forming process. As shown in Figure 7, materials with low elongation (<10%) could not deform sufficiently to form a proper joint and instead developed cracks, leading to an unacceptable connection.

Q2: The high-vacuum HPDC process had the highest tooling cost. How is this justified for mass production?

A2: The economic justification comes from the extremely low piece price and high production rate. According to Table 6, while the HPDC tooling cost is €400,000, the piece price is only €200. In contrast, sand casting has a piece price of €4000. For high-volume automotive production, the initial tooling investment is quickly amortized over thousands of units, making HPDC the most cost-effective method in the long run.

Q3: What is the significance of the T7 heat treatment applied to all samples?

A3: The T7 heat treatment, which involves solution heat treating followed by over-aging (stabilization), was applied to all samples to achieve a consistent and optimized state of mechanical properties. This process is crucial for improving the ductility and dimensional stability of the cast aluminum parts, which, as the study shows, directly enhances the performance and reliability of SPR connections.

Q4: The paper mentions a development process from TG0 to TG2. What do these stages represent in automotive engineering?

A4: These designations represent key milestones in the vehicle development process. TG0 typically corresponds to the initial concept or "Mule" phase, where basic feasibility is tested. TG1 aligns with the Design Validation (DV) phase, involving engineering prototypes for testing. TG2 represents the Product Validation (PV) phase, using production-intent parts and processes to finalize the design for mass production. The study used different casting processes best suited for the needs of each stage.

Q5: All three processes passed the X-ray inspection in the critical top area. Why then was the performance in the SPR test so different?

A5: X-ray inspection primarily identifies volumetric defects like porosity and shrinkage, ensuring internal soundness. However, it does not characterize the material's microstructure or mechanical properties like ductility. While all parts were internally sound, the HPDC part's superior, fine-grained microstructure (Figure 5) gave it the high elongation needed to withstand the severe plastic deformation of the SPR process, a property that the other processes could not deliver to the same degree.

Conclusion: Paving the Way for Higher Quality and Productivity

This comprehensive study highlights a critical lesson for modern automotive manufacturing: for structural components, the choice of an Aluminum Shock Tower Casting process must be guided by the demands of the entire production chain, especially assembly and joining. The research clearly demonstrates that while processes like investment and sand casting are invaluable for prototyping, high-vacuum HPDC is the champion for mass production. Its ability to deliver parts with superior ductility ensures the reliability of modern joining techniques like SPR, preventing costly failures and ensuring vehicle safety.

At CASTMAN, we are committed to applying the latest industry research to help our customers achieve higher productivity and quality. If the challenges discussed in this paper align with your operational goals, contact our engineering team to explore how these principles can be implemented in your components.

Copyright Information

This content is a summary and analysis based on the paper "Comparative Analysis of Three Casting Processes for Aluminum Alloy Shock Towers" by "Zhang You-guo, Wang Xue-feng, and Huang Zhi-gang".

Source: Foundry, 2019, Vol. 68, No. 10, pp. 1148-1154.

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